High energy accelerators typically operate with high vacuum, as do the RF sources that deliver the drive power to the accelerator cavities. Consequently, RF windows are required to maintain vacuum integrity during installation and removal of sources from the accelerator. RF windows are also required on the RF source to maintain internal vacuum. Typically, the RF power is transmitted in rectangular, circular or coaxial waveguide, so the RF windows between waveguide sections are required to function in the appropriate geometry. Most RF windows are fabricated from high purity alumina ceramic, and at high RF levels, the electric fields around discontinuities in the waveguide or on the surface of the RF window become high enough to cause multipactor, which is a resonant discharge caused by an avalanche of secondary electrons driven by the RF fields. The susceptibility for multipactor is a function of the RF power level, pulse width, RF frequency, secondary electron yield, DC charge on the RF window, and the surrounding gas environment. When multipactor occurs, energy is transferred from the RF wave to the cascading electrons, which energy is then deposited into the surface of the RF window. Failure of the RF window occurs when the thermo mechanical stresses which accumulate on the ceramic RF window exceed the yield strength of the RF window material. Often, this failure is catastrophic, resulting in breakage of the RF window and complete loss of vacuum integrity. One technique to address multipactor discharge is to sputter a 10-20 Angstrom thickness layer of a conductive coating on the surface of the RF window. Sputter deposition coatings utilize a physical deposition process where an evaporated metal condenses onto a surface as a film. Among the many problems of such film depositions are the poor adhesion between the substrate and the film, which relies on mechanical bonding at the surface of the substrate.
Accordingly, three important factors impact the window design. First, the window must support the high RF electric field without breakdown. Second, the window must be protected from multipactor to avoid surface damage and ultimately failure. Third, the RF window material must survive the thermal and mechanical stresses encountered during operation and exposure to atmospheric pressure. In configurations where the waveguide is pressurized with a dielectric gas, such as SF6, the window must adequately support the resultant differential pressure. Finite element analysis codes provide highly accurate simulation of the thermo mechanical stresses imposed on the RF window. A high reliability RF window should have adequate safety margins against failure for static pressures due to differential pressure and the thermal loads imposed when transmitting RF power. Thermal stresses depend on the amount of RF power absorbed during the RF transmission, which depends on the loss tangent of the material, and the thermal stresses which develop also depend on how the RF window is cooled. The ability to adequately manage the resultant thermal stresses depends on the yield strength of the material. The magnitude of the RF electric fields can be calculated analytically or simulated with time domain RF codes, such as the High Frequency Structure Simulator (HFSS) by Ansys (Canonsburg, Pa.). The susceptibility to arcing at the RF window depends on the geometry and the mode of the RF transmission. Most RF power is transmitted in fundamental mode through a waveguide. For a rectangular waveguide this is the TE10 mode, for circular waveguide this is the TE0n modes, and for a coaxial waveguide it is the TEM mode. Both TE and TEM modes impose electric fields parallel to the window surface and TEM modes impose electric fields perpendicular to the waveguide walls. Common failure mechanisms for an RF window positioned in the waveguide propagation path are electrical breakdown initiated at the waveguide-RF window joint, or surface flashover on the window surface. Arcing initiated at the waveguide-RF window joint is most prevalent, since this joint presents an interface involving four materials: the RF window, metallization of the RF window, a braze alloy, and the surrounding waveguide material. Any exposed edges or protrusions increase the local field gradient, generating an enhanced electric field which initiates the arcing which leads to breakdown. An alternative is to use an overmoded window with an axi-symmetric mode.
It is desired to provide an RF window which is resistant to multipactor, has a long usable life, and is suitable for use in a waveguide carrying RF power levels on the order of 100 MW and above.